A. Field of the Invention
The invention relates to compositions and methods for increasing the permeability of a subterranean formation. More particularly, the invention relates to acidizing compositions and methods for their application in subterranean formations comprising siliceous material.
B. Background
Acidizing of siliceous formations by injection of compositions referred to as mud acid is common practice in oilfield operations. As commonly understood, the expression “mud acid” refers to an aqueous base mixture formed by blending hydrofluoric acid (HF) and at least one of hydrochloric acid (HCl), acetic acid (C2H4O2), or formic acid (CH2O2), the most common mixture employed being formed from HF and HCl. Often, if acetic acid or formic acid are the acids combined with the HF, the mud acid is referred to as “organic mud acid”. As is well recognized in the art, the origin of the species in the acidizing solution or mixture is not critical, so that a “mud acid”, which might be formed by directly blending, e.g., HF and HCl, also is understood to include aqueous mixtures or solutions formed by mixing components which quickly react to form the desired ionic species in the solution or mixture. The ratios and amounts of the acids combined may vary over wide ranges, with the lower limits being more a matter of practicality rather than operability, and the upper limits being a matter of mutual solubility of the acids. Most typically, a mud acid is formed by combining about 3 to about 25 percent HCl and about 1 to about 10 percent HF, both percentages by weight, in aqueous solution, and is typically substantially free of other acidic species. As will be recognized by those skilled in the art, mud acids may also contain, and commonly do, one or more functional additives, such as inhibitors, diverting agents, and/or surfactants.
Although conventional treatments of siliceous clay containing formations with mud acids have generally proven effective for a short time, the improvements in production are frequently short lived. One explanation for this phenomenon is that the mud acid reacts rapidly with the subterranean formation in the vicinity of or near wellbore area, usually the first few inches around the wellbore, thus spending so rapidly that penetration deep into the subterranean formation is not achieved. Subsequently, fines in the subterranean formation migrate into the acidized near wellbore area and replug the area.
One solution to this problem is that taught in U.S. Pat. No. 3,828,854 (Templeton et al) and in the “Introduction” section of Society of Petroleum Engineers Paper No. 5153. The approach taken is the provision, down the wellbore, of a composition or solution which generates HF slowly, so that the solution is placed in contact with the subterranean formation before a significant amount of the HF is generated. The composition is a relatively high pH aqueous solution of a water soluble fluoride salt and at least one water reactive organic acid ester.
U.S. Pat. No. 2,300,393 (Ayers, Jr.) discloses treatment of subterranean formations with fluoboric acid, optionally containing small amounts of HF. Ayers, Jr. also teaches that the fluoboric acid treatment may be followed by HCl containing an inappreciable amount of hydrofluoric acid, or optionally, by a mixture of HCl and fluoboric acids. Again, U.S. Pat. No. 2,425,415 (Bond et al.) describes an acidizing procedure in which the subterranean formation is first contacted with a fluoboric acid solution which does not contain free HF, but which contains an excess of boric acid, followed by contact of the subterranean formation with aqueous fluoboric acid containing excess HF. U.S. Pat. No. 2,663,689 (Kingston el. al.) describes the use of boric acid in aqueous HCl—HF to avoid precipitation of insoluble fluoride salts and fluorosilicic acid. U.S. Pat. No. 4,151,878 (Thomas) is directed to the use of a conventional mud acidizing solution (HCl—HF), followed by fluoboric acid solution. The use of fluoboric acid as an overflush is believed to deter clay migration and thereby significantly reduce or delay production decline which is often otherwise encountered shortly after conventional mud acidizing treatments.
The Thomas patent also describes injection of a fluoboric acid solution, followed by mud acid (HCl—HF) solution. According to the patent, the technique may be used in formations which have a tendency to plug initially upon contact with mud acid, or with HCl commonly used as a preflush ahead of mud acid. When contacted initially with fluoboric acid, such subterranean formations show little or no plugging effects when subsequently treated with mud acid.
However, the Thomas patent does not specifically address formations containing zeolites and chlorites. As will be recognized by those skilled in the art, the use of traditional mud acid is not advisable in subterranean formations which comprise or contain HCl-sensitive materials, e.g., zeolite and chlorites. Additionally, fluoride in the mud acid is believed to bind with aluminum in the subterranean formation and promote deposition of hydrated silica, thereby causing plugging. For example, severe, damaging precipitation of aluminum fluorides during the HF reactions was discovered with formic-HF and acetic-HF fluid systems. See, C. E. Shuchart, et al., “Improved Success in Acid Stimulations with a New Organic-HF System,” SPE 36907 presented at 1996 European Petroleum Conference, Milan, Italy. To overcome this problem, Rogers et al. disclosed the use of citric acid as a chelating agent for aluminum to prevent such deposition or formation of hydrated silica gel. The optimum treatment formulation identified therein consisted of 10 percent citric acid and 1.5 percent HF acid, with no additives except corrosion inhibitor. One important disadvantage of this particular method is that the use of hydrofluoric acid primarily addresses damage or scaling in the initial few inches of the subterranean formation around the wellbore, as previously indicated.
Accordingly, there has been a need to extend acidization or stimulation treatment to deeper depths in the formation, e.g., up to a 3 to 5 feet radius from the wellbore, to avoid a rapid decline in production by stabilizing fines and precipitation of acidization products near the wellbore. The invention addresses this need.